An image forming apparatus, such as a printer and a photocopying machine, includes many rotative parts, such as a sheet conveyance roller and a photoreceptor drum. Stable driving of such rotative parts leads to smooth image formation operations, which results in favorable output images.
Generally, in such an image forming apparatus, the maximum capability of a heating means such as a heater, included in a fixing device thereof, is beforehand determined on the premise of the use of sheets of plain paper. Thus, in the case of forming an image on a sheet of special paper such as thick paper and an OHP sheet, under heating might be caused due to high thermal capacity of the paper. This can be a cause of a fixing failure.
To avoid this problem, a heater or the like with a high heating capability can be adopted in the fixing device. However, such a heater increases the power consumption contrary to the trend toward the energy saving, and inevitably increases the cost of the fixing device as well.
In view of the above, conventional technologies avoid a fixing failure by reducing the system speed without changing the heating capability of the heater and so on of the fixing device, to reduce the amount of heat taken away by the sheet per unit time.
In the case of performing such a system speed change, gears are generally used as a mechanism for transmitting power from the motor to each of the rotative parts. The system speed is changed by changing the gear ratio.
However, it is problematic that gears generate a relatively loud noise, which is annoying especially in a quiet office.
In view of this problem, a CVT (Continuously Variable Transmission) device using a belt drive is considered to be introduced as a speed changer for an image forming device.
According to this idea, each of the pulleys of the CVT device is divided into two segments in the middle of the axis of the pulley, and the V-shaped groove in the lateral surface of each pulley is configured such that the distance between the respective groove slopes of the divided segments is variable. This structure realizes smooth change of the ratio between the diameters of the pulleys, and thereby realizes smooth speed change. Such a structure provides comfortable office environment, because it does not generate gear noise.